11.  Thirdly, pollutions have caused that a lot of fresh water can't be used by human beings.

12.  We can turn sea water into fresh water by scientific means.

13.  Though seventy percent part of the earth is covered by the sea, the sea water is salty, what we need is fresh water .

14.  So the fresh water is very limited but some people think that the fresh water is never ending .

15.  But in fact fresh water is very limited.

16.  With the development of industry factories are using more and more fresh water.

17.  In addition the industrial waste water is let out into the rivers, polluting fresh water.

18.  So our human beings are facing a serious problem-- the shortage of fresh water.

19.  Remember this: Fresh water is the source of our lives!

20.  In general, people take it for granted that fresh water will never be limited to us.